Apparatus with capacitive probes for measuring the location and disposition of an interface between two media

ABSTRACT

Apparatus for measuring location and disposition of an interface between two media exhibiting a difference in dielectric constants or electric conductivity and including a column of pairs of capacitors arranged in a direction along which the location and disposition of the interface may vary, the media influencing the value of the capacitors. The capacitors of a pair having first common electrode configuration connected to an input line and second, individual, galvanically separated electrodes connected to a differential amplifier; electrical pulses are fed sequentially to the input lines and coupled to the differential amplifier through the pairs of capacitors. Imbalance on the differential amplifier output is referenced against pulse count reached when the imbalance occurs.

O United States Patent 1191 [111 3,777,257

Geisselmann Dec. 4, 1973 [54] APPARATUS WITH CAPACITIVE PROBES 2,754,4787/ 1956 Goldsmith 324/61 P FOR MEASURING THE LOCATION AND 3,181,5575/1965 Lannan, Jr... 324/61 R X 3,297,941 1/1967 Wolfendale 324/61 R XDISPOSITION OF AN INTERFACE 3,301,056 1/1967 Blanchard et a1... 324/61 RX BETWEEN Two MEDIA 3,631,430 12/1971 West 324/61 R x [75] Inventor,Heribert Geisselmann, 3,665,209 5 1972 Webb et a1 324/61 R xWohlen/Aargau, Switzerland P l [73] Assignee: Camille BauerMessinstruments 7333 g ggli g i i z if g Krawczcwlcz Aktiengesellschaft,Wohlcn/Aargau, y g Switzerland [57] ABSTRACT [22] F1led: May 2, 1972Apparatus for measuring location and disposition of [2]] Appl' N05249,634 an interface between two media exhibiting a differ- ForeignApplication Priority Data May 6, 1971 Switzerland 6742/71 US. Cl. 324/61R, 73/304 C Int. Cl G01r 27/26 Field of Search 324/61 R, 61 P;

References Cited UNITED STATES PATENTS 2/1972 Long 324/61 R 9/1967Johnston.... 73/304 C 1/1968 Hoenisch 73/304 C X ence in dielectricconstants or electric conductivity and including a column of pairs ofcapacitors arranged in a direction along which the location anddisposition of the interface may vary, the media influencing the valueof the capacitors. The capacitors of a pair having first commonelectrode configuration connected to an input line and second,individual, galvanically separated electrodes connected to adifferential amplifier;

electrical pulses are fed sequentially to the input lines and coupled tothe differential amplifier through the pairs of capacitors. Imbalance onthe differential amplifier output is referenced against pulse countreached when the imbalance occurs.

10 Claims, 2 Drawing Figures PATENTEUUEE 41915 J v 3.771257 sum 1 0F 2 IF-WJ PATENTED 5 SHEET 2 0F 2 I 4- BEE mmvw VENT E E QQ APPARATUS WITHCAPACITIVE 'PROBES FOR MEASURING THE LOCATION AND DISPOSITION OF ANINTERFACE BETWEEN TWO MEDIA BACKGROUND OF THE INVENTION The presentinvention relates to an apparatus for detecting and measuring theposition and location of one or more interfaces between different media;more particularly, the invention relates to apparatus which iscapacitively responsive to different dielectric constants of two media;the media itself may be different materials and/or they may appear indifferent phases. In other words, the invention includes detection ofthe interface between the gas and liquid phases or the interface betweensolid and gas phases or the interface between the liquid and solidphases of, possibly the same or different materials. The invention alsoincludes the detection of the interface between the same phases ofmaterials with different electric properties.

Capacitively measuring the location of an interface usually involves theresponse to the difference in capacitance of capacitors when one or theother material serves as dielectric between the respective capacitorelectrodes. Capacitive probes of this type usually operate on basis ofdevelopment of analog signals representing this difference in dielectricconstants. However, these probes are dependent on the type, nature andcondition of the material involved and on other relevant characteristicsthereof including all materials with which the electrodes come intocontact, or are within the effective range for influencing capacitance.

Also shape, type and other features of the container for the interfacedefining media are operative but undesirably variable parameters for themeasuring process. As the detecting equipment is to operate as nullinstrument accurate compensation of'all these parameters is needed.Drift and temperature dependency, possible deposition of impurities andother time variable factors complicate matters significantly.

DESCRIPTION OF THE INVENTION It is an object of the present invention toprovide an apparatus for capacitively measuring location and position ofan interface between two media which operates independently from thematerial and physical characteristics of the media involved to theextent that, as much as possible any difference in electric constants ofthe media across the interface is the only operative parameter. Theelectric constants of the individual media, including possiblevariations for any reason, should not become operating parameters bythemselves, including operative modifications thereof as resulting fromthe measuring vessel etc. Therefor, the apparatus in accordance with theinvention should require compensation, trimming, adjustment etc. aslittle as possible.

In accordance with the preferred embodiment of the invention, thecapacitive measuring apparatus is to include a capacitor column (d.c.separated from the medium by a layer of insulator) wherein respectivetwo adjacent capacitors, being spaced-apart in direction and havingdisplacement component transverse to the orientation of the interface tobe detected, have one common (or common potential) electrodeconfiguration and two individual and separated electrodes. 'The commonelectrode or electrodes of such a pair of capacitors receive pulses andthe two individual electrodes feed to the two inputs of a differentialamplifier.

The several electrodes are constructed so that these two capacitors feedsimilar inputs to the differential amplifier if the same medium acts asdielectric or conductor in both of them; if the interface separatesthem, the differential amplifier receives different inputs which is usedas indication for the location of the interface. It can be seen thateach pulse is a.c. coupled through the two capacitors of a pair to thetwo inputs of the differential amplifier, and different dielectrics orconductors in the two capacitors distort the otherwise similar signalcoupling so as to imbalance the differential amplifier input.

Each pair of electrodes, as defined by and having a common inputelectrode configuration connects to a separate sample signal input line.There are as many input lines as there are pairs of capacitors. Thesesample signal input lines are scanned sequentially by pulses of a train,while alternating capacitors along the col umn and pertaining todifferent pairs, have their individual electrodes connected to the sameinput of the samedifferential amplifier. In other words, all pairs ofcapacitors have one electrode connected to one common pick-up electrode,and the respective other pickup electrodes of all pairs are likewiseinterconnected for establishing a second common pick-up electrode, sothat the pick-up electrodes of all pairs feed into the same two inputsof one differential amplifier. These two common pick-up electrodes havepreferably comb-' like configuration, and the input electrodes aredisposed in-between the fingers of the combs.

Sequential pulses of the train are set sequentially (distributed) intothe inputs corresponding to scanning the column of capacitors from oneend to the other.

The number of pulses is counted as long as the differen-' tial amplifiersenses equality of inputs. The count number reached when signalinequality is sensed on the amplifier is indicative of the interfacelocation.

It can also be seen that the invention combines digital and analogtechniques in that basically digital pulses are used for acquisitionpurpose, but each pulse as an a.c. signals is modulated by the twocapacitors of a pair and in the case of unequal modulation, thedifferential amplifier responds.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention and further objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawings in which:

FIG. 1 illustrates the development of a cylindrical electrode patternestablishing the column of capacitors, e.g., the inner wall of acylindrical sampling vessel; and

FIG. 2 is a block diagram for digitally indicating the position of oneor two interfaces between different media.

DESCRIPTION OF THE DRAWINGS Proceeding now to the detailed descriptionof the drawings, FIG. 1 illustrates, for example, the planar developedand projected interior surface of a cylindrical tube made of insulatingmaterial with copper plated electrodes. The figure could also beconstrued as a printed circuit foil made, for example, on a flexiblebase and to be used as lining in a sampling tube. In either case suchcontainer is presumed to hold two media, separated by an interfaceschematically represented by the dash line. Variations in the interfacelocation occur transversely to that interface defining line. Theelectrode pattern as illustrated may additionally be covered by alining, coating or hose (in the tubular configuration) made of amaterial commonly known as Teflon.

The electrode pattern includes two comb-like configurations, Al and A2,having appropriately provided lead-in electrodes. These two electrodesare galvanically separated from each other. The combs face each other(in a plane) with the respective prongs or fingers, but at a phase shiftof one or of more finger spacing. Additional electrodes are disposedin-between two fingers of each comb; electrodes E11, E21, E51, are sodisposed as to electrode Al, and there are provided electrodes E12, E22,E52, between the fingers of electrode A2. As a consequence, tencapacitors are defined; each of the electrodes Eij (i l, 5;j= l or 2)defines one capacitor together with the respective adjacent two fingersof electrode configuration A1 or A2 as the case may be. Effectivecapacitance is completed in each instance by the dielectric materialclosest to the respective electrodes.

Electrodes Eij are interconnected in pairs and that interconnectionestablishes a common electrode configuration. Thus, electrodes E11 andE12 form a first common configuration. They are interconnected by simplep.c. line, but could be of unitary construction. The otherinterconnected pairs are E21-E22; E31- E32; E4l-E42 and E5l-E52. Thesefive interconnected electrodes respectively connect additionally to thefive input lines E1, E5. Conceivably, these lines may be part of p.c.line configurations on the base. Accordingly, there are five pairs ofcapacitors, each pair having a common input electrode configuration forits two capacitors, Ei-Eil-EIZ, i= 1, 5 and having additionally twooutput or pick-up electrodes defined by the respectively closest fingersof the electrode combs A1 and A2.

That there are five such capacitors is totally arbitrary, the number hasbeen chosen for illustrative purposes only. The actual number used inany instance depends on the displacement range for the interface to belocated and on the resolution of measurement. A multiple combarrangement with appropriate staggering can also be provided.

The effective areas of the several electrodes have been chosen so thatthe capacity between any electrode Eil and the adjacent fingerconfiguration of electrode A] is the same as between the correspondingelectrode Ei2 (of the same pair), and the adjacent finger configurationof electrode A2, provided the electrodes face the same electric couplingmaterial (i.e., material of the same electric constant to be exact). Inthe illustrated, assumed case, Ell-A1 and E12-A2; E2l-Al and E22- A2,are two pairs of capacitors of equal capacity. However, the capacitorE3l-A1 has capacitance different from the capacitance of capacitorE32-A2, as the former is above and the latter is below the interfacebetween two media and, thus, they have different electric coupling to Arespective to A The capacitors E41-A1 and E42-A2 are again similar, soare the capacitors E5l-Al and E52-A2.

If now an a.c. signal is applied to line El, pick-up electrodes A1 andA2 will develop similar signals. The

same is true when the signal is applied to line E2. As the signal isapplied to line E4 or E5, each of the pick-up and sampling electrodes A1and A2 develops a signal that is different from before, but again theyare similar among themselves, i.e., there is no difference in signallevel on electrodes Al and A2 in each of these cases. However, as thesampling signal is applied to line E3, the pick-up electrodes developdifferent signal levels. How this is being used will be explained nextwith reference to FIG. 2.

Reference numeral 1 in FIG. 2 refers to the electrode assembly andcolumn as shown in FIG. 1 and may include the container having theinterface to be located. Whatever the configuration, lines E1 to E5denote the five inputs, and Al and A2 are the outputs as derived fromthe pick-up electrodes of like designation.

A pulse generator 2 provides a train of pulses and feeds them to acommon or central input of a signal distributor 3. The signaldistributor 3 is shown as step switch, but could have electronicconfiguration (shift register with output gates or the like). Signaldistributor 3 has six outputs, five of which are connected respectivelyto the input lines E1 to E5 of the capacitor column. The sixth output ofdistributor 3 (line 3a) connects to various circuits to be describedbelow for marking completion of a scanning and sampling cycle andbeginning of another one. Therefor, distributor 3 distributes pulses tolines E1 to E5 and in the sixth switching position a reset pulse ispassed into the systern.

The distributor 3, when constructed as step switch, is operated forstepping by a step motor 6. The motor 6 receives also the pulses fromgenerator 2, but via a delay circuit 5. The circuit 5 delays each pulsefrom generator 2 by a period equal to about half the pulse rate period.Therefor, motor 6 receives a trigger or actuator pulse inbetween twosequential pulses as applied to distributor 3. The distributor will passa pulse into one of its six output lines, and during the pause to thenext pulse the distributor is stepped or shifted by one position, etc.Six pulses, thus, mark one complete measuring cycle for the system.

The individual pulses, thus, serve as a.c. input for each capacitorpair, and the capacitors of the pair serve as a.c.-coupler to the twoinputs of a differential amplifier 8, which are connected to the twopick-up electrodes Al and A2 accordingly. All pulses of the pulse trainare fed to the clock input of a counter 7 via a gate 4. The gate 4 isunder control of a flip-flop 24. The flipfiop is set when thedistributor 3 passes a pulse into line 3a which is the cycle-completionpulse of the system, now setting up the system for a new sampling cycle.The same pulses in line 3a pass into the reset input 7a of counter 7,resetting it to zero count state for each new cycle.

Flip-flop 24 is reset by a pulse in a line 9a which connects to theoutput of a pulse shaper 9 receiving as input the output of differentialamplifier 8. As long as flip-flop 24 is in the set state, gate 4 passespulses from generator 2 to the counter 7 which tallies the pulses soapplied. Once the flip-flop is reset by response of amplifier 8, gate 4is blocked for the remainder of the cycle.

In operation, a cycle begins when distributor 3 sets a pulse fromgenerator 2 into line 3a, setting flip-flop 24 for opening gate 4 andresetting counter 7 to count state zero. The same pulse as far asproduction by generator 2 is concerned, is delayed in 5 and operatesmotor 6 for shifting the distributor into the next position. Clockwisemotion is presumed as indicated by the arrow. Thus, the next clock pulseis set into line El. If

the capacitors Ell-A1 and E12-A2 (FIG. 1) are adjacent the same medium,two similar signals pass to the two inputs of amplifier 8 and no outputis produced. Pulse shaper 9 may have a particular response level fornoise suppression to compensate for any low level transients. Thus, aflip-flop reset pulse is not produced. As gate 4 is open, the samesample pulse passes and increments counter 7 by one. Still the samepulse is delayed in 5 and soon causes distributor 3 to shift to the nextposition.

The next pulse passes into line E2, and in the illustratedrepresentative example this pulse as applied to line E2, is effective incapacitors E21-Al and E22-A2, which have also the same circumstances inthis instance. Thus, differential amplifier 8 receives balanced inputsagain, and pulse shaper 9 does not respond. The flip-flop 24 remainsset, gate 4 remains open and counter 7 is duly incremented by one tocount state two. The same pulse as delayed shifts distributor 3 so thatthe next pulse from generator 2 passes to line E3 for the thirdcapacitor pair in the column, but also, through still open gate 4 tocounter 7 for the counter to assume count state three. On the otherhand, capacitor E31-A1 is exposed to a medium with electric propertiesdifferent to the medium effective in capacitor E32-A2 as the interfaceis presumed to pass between them. Accordingly, this third sample pulseis picked up by electrodes Al and A2 at different levels; the inputs ofamplifier 8 do not balance and an output pulse is produced by shaper 9.A slight delay here may prevent a race condition so that flip-flop 24 isreset only after this pulse has passed through the gate 4. However, ifflip-flop and counter operate on falling clock trigger, no problemexists and the pulse is still properly counted as the closing gate mayproduce the falling edge.

The next pulses pass through the system but closed gate 4 ignores them.The counter, thus, indicates the number of pulses, i.e., the number ofsteps for scanning the capacitor pairs of column 1 until the one pairwas reached in which the interface served as divider to imbalance thecharacteristics of the capacitors of the pair. in the assumed example,it was the third pair and its location in the column represents directlythe location of the interface.

Operation continues until the distributor 3 has completed a cyclewhereupon the system resets to zero state and begins the next measuringcycle. The counter reset pulse at input 7a may be effective only in thecounter proper as far as resetting is concerned. The counter proper maybe a shift register, binary counter or the like. Unit 7 may include ahold section controlling directly an indicating portion (mechanical ornixie tubes etc.), and the counter reset pulse may serve as transfercontrol signal to the hold section for updating the content thereof. Thehold section may be a register and the pulse at input 7a opens parallelgates between counter proper and register. Thus, the indicator portionof the counter will change only if in -between cycles there is a changein interface position. Otherwise the count state indicated isdynamically maintained. If the system is halted for any reason, the lastindication may still remain.

In case the interface position is to be indicated as analog value, anintegrator 10 may be connected to the output of gate 4 for integratingthe pulses passing through and now presumed to have constant amplitudeand duration (or at least constant time voltage integral). The pulse inline 3a is applied to a reset input 10a of the integrator to reset theintegrator with the beginning of each cycle. An instrument 11 isconnected to the output of the integrator 10. A hold circuit may beinterposed unless the response time of indicator 11 is slower than thecycle rate of the distributor 3.

It can be seen that the indicator for the location and disposition of aninterface relates directly to the disposition of the several pairs ofcapacitors. There is no inherent necessity that these locations bear anyspatial relation to each other. Each number may be interpreted as alocation identification number for the interface to be detected.However, the system can be used as direct indication of interfaceposition on a linear scale, in which case the pairs of capacitors mustbe regularly spaced. In a different version, the system may, forexample, be used to indicate the filling state in a container ofirregular volume contour, for example, a cylindrical container or tanklying down. The surface level height therein is not linearly related tovolume, but by irregularly spacing the capacitors, e.g. on one insidefront end of the container with denser spacing in the middle thannear-top and bottom, such scale distortion can be made to compensate thenon-linear relation between level height and volume.

Some of the elements shown in FIG. 2 have not yet been explained. Theyrelate to the fact that the system as explained thus far, can besupplemented by circuitry for detecting the locations of more than oneinterface, e.g., in a multi strata configuration of several media.

Reference numeral 12 refers to another gate and flipflop circuitassembly equivalent to elements 4 and 24 above. The gate in circuit 12receives also the pulses from generator 2 and passes them to a counter13 unless inhibited. The reset input 13a of this counter receives thecycle pulse in line 3a, and still the same signal sets the flip flop incircuit 12 via input 12a thereof.

The output of pulse shaper 9 operates a toggle flipflop 14. It can beseen that for two-interfaces the second response of differentialamplifier 8 is indicative of the location of the second interface.Toggle flip-flop l4 suppresses the first response and only the secondresponse is used to reset the flip-flop in gating circuit 12 via input12b thereof. Therefor, counter 7 is halted for the first interface andcounter 13 for the second interface, and the respective count numbersrepresent their locations. That the flip-flop 24 receives a second resetinput on detection of the second interface does not influence thecircuit. Additional counter-gate assemblies can be added with anothercounter to be used for counting the number of responses of differentialamplifier 8, so that the several counters are halted in proper sequence,each representing the location of one interface. Also, analog indicationof the type 10-11 can be used in addition or in lieu of digital countingin each case.

The invention is not limited to the embodiments described above but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention are intended to be included.

I claim:

1. Apparatus for measuring location and disposition of an interfacebetween two media exhibiting a difference in electric constants andincluding a column of capacitors arranged in a direction along which thelocation and disposition of the interface may vary, the electricproperties of the media influence the capacitors of the column, theimprovement comprising:

the capacitors of the column arranged in pairs, the

capacitors of a pair spaced-apart in said direction;

the capacitors of a pair having a first electrode for a first capacitorof the pair, a second electrode for a second capacitor of the pair and athird electrode configuration being the respective counter electrode forboth capacitors of the pair, the first and second electrodes and thethird of the pair all being galvanically separated from each other, thefirst, second and third electrodes of each capacitor pair beingstationary relative to each other, the third common potential electrodeconfigurations of all pairs being galvanically separated from eachother;

a plurality of input lines respectively connected to the third electrodeconfiguration in the pairs;

first means for feeding sequentially pulses to the input lines of theplurality to be sequentially effective in the pairs of capacitors, eachpulse effective in both capacitors of a pair;

second means including two output lines respectively connected to thesecond individual electrodes of each pair for establishing two pick-upelectrode configurations, whereby all of the first electrodes ofrespectively all first capacitors are interconnected and connected toone of the two output lines, and all the other second electrodesrespectively of all the second capacitors are interconnected andconnected to the other one of the two output lines; i f

a single differential amplifier having two inputs connected respectivelyto the two output lines; and

third means connected to the first means and to the differentialamplifier for deriving therefrom representation of the interfacelocation and disposition in relation to the capacitor column.

2. Apparatus as in claim 1, wherein the first means includes a pulsegenerator and a pulse distributor feeding the pulses from the generatorsequentially into the input lines of the plurality.

3. Apparatus as in claim 2, wherein the third means includes a counterconnected for counting pulses from the pulse generator, furtherconnected to halt counting when the differential amplifier sensesdifferent inputs as applied by the two output lines.

4. Apparatus as in claim 3, wherein the pulse distributor operatescyclically and means are provided for restarting the counting operationfor each distribution cycle.

5. Apparatus as in claim 2, wherein the third means includes anintegrator connected for integrating pulses from the generator until thedifferential amplifier senses different inputs as applied by the twooutput lines.

6. Apparatus as in claim 2,' wherein the third means is responsive toplural sequential responses of the differential amplifier to inequalityof inputs on the two output lines and provides for separatemanifestations representing location and disposition of differentinterfaces.

7. Apparatus as in claim 1, wherein the several electrodes are providedas printed circuit platings on an insulative backing.

8. Apparatus as in claim 7, wherein the printed circuit platings arecovered by an insulative layer.

9. Apparatus as in claim 7, wherein the backing has tubularconfiguration.

10. Apparatus as in claim 1, wherein the first and second electrodes ofthe pair and including the second means are provided by two comb-likeelectrode configuration having fingers, and electrodes pertaining to thefirst common electrode configuration are disposed between the fingers.

1. Apparatus for measuring location and disposition of an interfacebetween two media exhibiting a difference in electric constants andincluding a column of capacitors arranged in a direction along which thelocation and disposition of the interface may vary, the electricproperties of the media influence the capacitors of the column, theimprovement comprising: the capacitors of the column arranged in pairs,the capacitors of a pair spaced-apart in said direction; the capacitorsof a pair having a first electrode for a first capacitor of the pair, asecond electrode for a second capacitor of the pair and a thirdelectrode configuration being the respective counter electrode for bothcapacitors of the pair, the first and second electrodes and the third ofthe pair all being galvanically separated from each other, the first,second and third electrodes of each capacitor pair being stationaryrelative to each other, the third common potential electrodeconfigurations of all pairs being galvanically separated from eachother; a plurality of input lines respectively connected to the thirdelectrode configuration in the pairs; first means for feedingsequentially pulses to the input lines of the plurality to besequentially effective in the pairs of capacitors, each pulse effectivein both capacitors of a pair; second means including two output linesrespectively connected to the second individual electrodeS of each pairfor establishing two pick-up electrode configurations, whereby all ofthe first electrodes of respectively all first capacitors areinterconnected and connected to one of the two output lines, and all theother second electrodes respectively of all the second capacitors areinterconnected and connected to the other one of the two output lines; asingle differential amplifier having two inputs connected respectivelyto the two output lines; and third means connected to the first meansand to the differential amplifier for deriving therefrom representationof the interface location and disposition in relation to the capacitorcolumn.
 2. Apparatus as in claim 1, wherein the first means includes apulse generator and a pulse distributor feeding the pulses from thegenerator sequentially into the input lines of the plurality. 3.Apparatus as in claim 2, wherein the third means includes a counterconnected for counting pulses from the pulse generator, furtherconnected to halt counting when the differential amplifier sensesdifferent inputs as applied by the two output lines.
 4. Apparatus as inclaim 3, wherein the pulse distributor operates cyclically and means areprovided for restarting the counting operation for each distributioncycle.
 5. Apparatus as in claim 2, wherein the third means includes anintegrator connected for integrating pulses from the generator until thedifferential amplifier senses different inputs as applied by the twooutput lines.
 6. Apparatus as in claim 2, wherein the third means isresponsive to plural sequential responses of the differential amplifierto inequality of inputs on the two output lines and provides forseparate manifestations representing location and disposition ofdifferent interfaces.
 7. Apparatus as in claim 1, wherein the severalelectrodes are provided as printed circuit platings on an insulativebacking.
 8. Apparatus as in claim 7, wherein the printed circuitplatings are covered by an insulative layer.
 9. Apparatus as in claim 7,wherein the backing has tubular configuration.
 10. Apparatus as in claim1, wherein the first and second electrodes of the pair and including thesecond means are provided by two comb-like electrode configurationhaving fingers, and electrodes pertaining to the first common electrodeconfiguration are disposed between the fingers.